In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, future generations wireless communications networks are expected to provide ubiquitous high data-rate coverage. Currently emerging standards, such as the 3rd Generation Partnership Project (3GPP) Long Term Evolutional Advanced (LTE-Advanced), are targeted to support up to 1 Gbps in the downlink (i.e., from the network nodes to the wireless devices) and 500 Mbps in the uplink (i.e., from the wireless devices to the network nodes). In general terms, achieving such data rates requires an efficient use of the available resources. With multiple antennas at the transmitter and/or the receiver (such as at the network node and/or the wireless devices), it is possible to exploit the spatial degrees of freedom offered by multipath fading inside the wireless channel between the transmitter and the receiver in order to provide an increase in the data rates and reliability of wireless transmission.
In conventional cellular networks, network nodes, such as radio base stations, are typically installed at a height significantly above the wireless devices, also referred to as user equipment (UE), to be served, and the cell radius (defining the border within which the network node provides coverage to the wireless devices) is much larger than the height difference between the transmitter and receiver nodes. In such situations, most of the multipath fading takes place in the azimuth plane of the wireless channel. Hence, a fixed antenna pattern is commonly employed in the elevation dimension.
In current wireless communications networks smaller cell sizes are becoming more common and the wireless devices may be distributed in both the azimuth and elevation domains, sometimes even above the height where the network nodes are placed. Therefore, the wireless propagation channel between the network node and each served wireless device becomes a truly three-dimensional (3D) propagation environment where the multipath fading occurs both in the azimuth and elevation domains.
With the emergence of active antenna systems (AAS), active transceivers (transmitters and receivers) are integrated with each, or a group (i.e., subarray) of, radiating elements inside the antenna radome. This enables adaptive weighting of individual antenna elements or subarrays using baseband processing. A two-dimensional (2D) planar active antenna array provides freedom in controlling radiation patterns in both the elevation and azimuth domains. This is different from legacy network node antennas that use horizontal linear arrays with a fixed radiation pattern in the elevation domain.
One example of 2D spatial processing capability of an AAS is to perform user-specific 2D beamforming. In this approach the antenna array at the network node adaptively controls individual beams in the elevation and/or azimuth planes to match the propagation channel of the wireless device of interest. User-specific 2D beamforming may enhance link throughput and extend both downlink and uplink coverage.
A linear antenna is suitable when user-specific beamforming is to be performed in one dimension only. However, some wireless devices may benefit from beamforming in azimuth whereas other wireless devices may benefit from beamforming in elevation.
In general terms, proper selection of beamforming weights in the network node based on feedback from the wireless device is a matter of not only the number of downlink reference signals and the codebook which is used for evaluation and/or basis for the feedback information but also that there is a match between the antenna topology and the codebook design. The latter is typically the case for single column (or row) dual polarized antennas but typically not for two-dimensional antennas with beamforming capabilities in two dimensions.
Hence, there is still a need for an improved user-specific beam forming.